


A Knock Knock From Yourself

by FlyingDuckMaster



Category: Undertale (Video Game)
Genre: Alterfell kind of?, Alternate Universe, Alternate Universe - Underfell, Alternate Universe - Underswap, Blue - Freeform, Dimension Travel, Edge - Freeform, It's Underfell, M/M, Red - Freeform, So expect less than pleasant issues, Stretch, Underfell Papyrus, Underfell Sans, Underswap Papyrus, Underswap Sans, honeymustard - Freeform, platonic Cherryberry
Language: English
Status: In-Progress
Published: 2018-04-04
Updated: 2018-04-04
Packaged: 2019-04-18 04:10:35
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings, Graphic Depictions Of Violence
Chapters: 1
Words: 575
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14204754
Author URL: https://archiveofourown.org/users/FlyingDuckMaster/pseuds/FlyingDuckMaster
Summary: Underswap Sans gets transported to Underfell. Needless to say, he is going to have a very bad time. A man who resembles Papyrus is hell-bent on hunting him down for resembling his missing brother.Blue has no idea what happened to the Sans of this world, until he reached the Ruins and knocked on that enormous door.





	A Knock Knock From Yourself

**Underfell Prologue**

"If a human child ever comes through this door... could you please, please promise something."

Those words...

"Watch over them, and protect them, will you not?"

Those words were the last Sans ever heard from the lady behind the door.

Soon, true to her words, a child came through the door. They were shivering in fear and covered in dust. Sans wasn't blind nor stupid, he knew who the dust belonged to. No matter how many times he came back to the door, the lady never answered. Not that he really needed to, the guilt on the child's face was telling enough.

Even so, he lead the child towards the barrier. He helped them avoid confrontation from Papyrus, Undyne, Asgore, and the watchful cameras of Alphys. Every second he wished he could go back and refuse that promise, but he owes the lady his life. She was one of the only joys in his life that he ever looked forward to. If she did not exist... well... maybe he would have continued with that suicide plan the day he met her.

Turns out, the lady was a boss monster, and the human passed through the barrier no problem. It was as if nothing happened, and time in the underground went on.

Just that the lady was missing.

Sans sighed, borrowing himself against the door. He missed the lady. He cared for her. She was the only nice monster he ever met under this hellhole, and now she is gone. Forever.

One day he went up to the door and took a leap of faith. If the child went through these doors after killing the lady, than that must mean the doors were never locked after their arrival. His theory was correct; just a light push was all it took for the doors to open.

From then on, he was a little lost at what he should do. Should he report it? Or should he just go back and pretend this never happened.

Going back would entail these ruins being taken over. The lady behind the door wouldn't have liked that. Of course, he could fully ignore all her wishes simply because she's dead, but despite everything this world tries to force on him, he still has a pathetic morality that makes him too soft for survival.

And if he does report the ruins despite his morals...

Everything would still be the same.

As much as he loves his brother, he had enough of their.... less than ideal relationship. Papyrus could care less about him, and frankly, Papyrus would also be better off without Sans.

Nobody liked him anyway. He was just the runt one hp brother of The Great and Terrible Papyrus. He had no title to his name, he wasn't even considered a monster. His status was so abysmal, he was treated less like a monster and more as a pet. Even if he was quick to anger, he was incredibly patient with his brother. But patience would only bring him so far, and his patience had already been shredded and stomped on a long time ago.

Sans entered the Ruins.

The ironic thing is, he always wanted to be free all his life. And finally, he can become free, it only took locking himself away from this world for it to happen.

The doors of the Ruins were sealed tight once again, and wouldn't be opened for a very long time.

**Author's Note:**

> Alright, just to clear some things up. The human killed Toriel and went through the door where they met up with Sans. The camera Alphys had on the Ruins were broken by an unknown force. (Which really was Sans because he wants to avoid the confrontation they would have if they find out he has been coming to the Ruins everyday.) Sans stealthily lead the human child to the barrier (aka teleportation for most of the trip) and let the human go through the barrier. Because the child already killed Toriel, who's a boss monster (And kept her soul, I guess?), they went through the barrier pretty easily and no one found out. 
> 
> AKA, how to speed run Undertale. Or is it Underfell?


End file.
